1. Field of the Invention
The present invention relates to a boundary acoustic wave device used as, for example, resonators and filters. In particular, the present invention relates to a boundary acoustic wave device including an electrode which is disposed between a first medium and a second medium, and a third medium and a fourth medium are laminated on the second medium.
2. Description of the Related Art
In recent years, boundary acoustic wave devices have been used as resonators and band pass filters because the package structures thereof can be simplified.
WO2004-070946 discloses a boundary acoustic wave device, wherein the electromechanical coefficient is relatively large, the propagation loss and the power flow angle are relatively small, and the temperature coefficient of resonant frequency TCF is within an appropriate range. Here, an IDT electrode is disposed at an interface between a first medium made of a piezoelectric substrate and a second medium made of a SiO2 film. Furthermore, the electromechanical coefficient and the temperature characteristics can be adjusted by adjusting the crystal orientation of a piezoelectric single crystal used as the piezoelectric substrate, the material defining the IDT electrode, the film thickness, and the electrode finger pitch.
WO2005-093949 discloses a boundary acoustic wave device 101 schematically shown in FIG. 12. In the boundary acoustic wave device 101, the IDT electrode 115 is disposed at the interface between the first medium 111 made of a Y-cut X-propagation LiNbO3 substrate and the second medium 112 made of a SiO2 film. Furthermore, a third medium 113 made of a polycrystalline Si layer and a fourth medium 114 made of a SiO2 film are laminated on the second medium 112 in that order.
Here, WO2005-093949 describes that frequency adjustment can be performed by laminating the second medium 112 to the fourth medium 114. That is, the electrode 115 is disposed between the first medium 111 having a film thickness of H1 and the second medium 112 having a film thickness of H2, and the third medium 113 having a film thickness of H3 is laminated, so that a laminate is obtained. The frequency control is conducted at this laminate stage. Then, the fourth medium 114 having a film thickness of H4 is laminated on the third medium 113. In the boundary acoustic wave device 101, the energy of the boundary wave is as shown in the right-hand portion of FIG. 12. That is, in the fourth medium, the energy is distributed to only a small portion. Therefore, when the frequency control is performed at the above-described laminate stage and variations in frequency are reduced significantly, even when the fourth medium 114 is formed, it is possible to reduce variations in frequency.
In the boundary acoustic wave device described in WO2004-070946, when the substrate crystal orientation of the piezoelectric single crystal defining the piezoelectric substrate and the configuration of the IDT electrode are determined, the temperature coefficient of resonant frequency TCF and the temperature coefficient of delay time TCD are automatically determined. Therefore, it is difficult to obtain a boundary acoustic wave device having desired temperature characteristics.
On the other hand, in the boundary acoustic wave device described in WO2005-093949, the frequency adjustment in the production stage can easily be performed by laminating the second medium to the fourth medium, as described above. Therefore, the boundary acoustic wave device exhibiting reduced variations in frequency can be provided. However, as is clear from the temperature coefficient of resonant frequency TCF shown in FIG. 10 in WO2005-093949, the temperature coefficient of resonant frequency TCF may deteriorate because the above-described third medium layer made of a polycrystalline Si layer is laminated.